1. Technical Field
This application claims the benefit of priority No. KR2012-0149326 filed on Dec. 20, 2012.
The present invention relates to a gasification process for co-producing synthesis gas and high-grade (high-quality) coal from water-containing coal. More particularly, the present invention relates to a gasification process in which a dryer for drying water-containing coal (feedstock) using the heat of synthesis gas is disposed in front of a gasifier, whereby dried coal can be easily transferred to the gasifier by a non-mechanical valve-type transfer means, and synthesis gas and high-quality coal containing a very small amount of water can be co-produced, and to an integrated drying and gasification system for realizing the same.
2. Description of the Related Art
Generally, a gasification process is a series of processes for converting a carbonaceous feedstock such as coal, biomass or the like into synthesis gas containing hydrogen and carbon monoxide as major components using a gasification reaction under the supply of a gasifying agent (for example, oxygen, steam, carbon dioxide or a mixture thereof). In this case, the term “synthesis gas” generally means a gas mixture including CO and H2. Examples of typical reaction mechanisms of such a gasification process are represented by the following Reaction Formulae 1 to 3.C+½O2→CO(partial oxidation)  [Reaction Formula 1]C+H2O→CO+H2(steam reforming)  [Reaction Formula 2]C+CO2→2CO(carbon dioxide reforming)  [Reaction Formula 3]
Recently, a gasification process has been diversified into various kinds of technologies for producing various compounds, and application fields thereof have spread in order to produce various products including electric power. Specifically, it is known that a gasification process can be applied to power generation, ammonia preparation, oil refining and the like using hydrogen and carbon monoxide included in synthesis gas which is a main product of a gasification process; that it can be used to produce diesel oil, jet oil, lubricant base oil, naphtha and the like using synthesis gas as a feedstock of Fischer-Tropsch reaction represented by the following Reaction Formula 4; and that it can be used to produce high value-added chemical materials such as acetic acid, olefin, dimethyl ether, aldehyde, fuel, additives and the like using methanol prepared from synthesis gas as represented by the following Reaction Formula 5. In relation to this gasification process, in the case of Fischer-Tropsch process and methanol synthesis process, it is preferred that the ratio of carbon monoxide to hydrogen be about 1:2.nCO+2nH2→CnH2n+nH2O  [Reaction Formula 4]CO+2H2→CH3OH  [Reaction Formula 5]
However, in the case of synthesis gas obtained by the steam reforming reaction represented by Reaction Formula 2 above and the carbon dioxide reforming reaction represented by Reaction Formula 3 above, the ratio of carbon monoxide to hydrogen does not reach 1:2. Therefore, generally, the ratio of carbon monoxide to hydrogen may be adjusted to about 1:2 by performing the water-gas-shift reaction of a reaction product obtained after a steam reforming reaction, a partial oxidation reaction and/or a carbon monoxide reforming reaction or by additionally supplying hydrogen to the reaction product.CO+H2O→CO2+H2  [Reaction Formula 6]
Meanwhile, since coal, which a typical feedstock in a gasification process, is distributed in large quantities over a wide area throughout the world, it is attracting considerable attention again as a fuel source that can replace petroleum that is commonly used at the present time. Further, since biomass, which has recently attracted attention, can also be converted into a basic fraction of various kinds of fuels and platform compounds by various treatment processes, there is a known technology of applying biomass as a feedstock in a gasification reaction.
However, the above-mentioned feedstock in a gasification process contains a large amount of water. For example, coals are classified into high-grade coal and low-grade coal depending on the properties thereof, such as heating value, water content, impurities, etc. Here, it is known that the amount of low-grade coal (brown coal, etc.) reserves occupies about 45% of the total amount of coal reserves throughout the world. Particularly, when the content of water in coal is high, the coal becomes poor in terms of storability, treatability, freight and the like, so a large amount of energy is consumed to remove water from the coal prior to a gasification reaction.
In relation to this, U.S. Pat. Nos. 5,695,532 and 5,685,138 disclose technologies for using the heat of synthesis gas to dry a water-containing carbonaceous feedstock by disposing a dryer in front of a gasification reactor and recycling the high-temperature synthesis gas generated from the gasification reactor into the dryer.
However, in order to supply the dried feedstock into an operating gasifier at high pressure, it is required that the supply pressure of the dried feedstock is higher than the operating pressure of the gasifier. Conventionally, in order to supply the feedstock (dry coal) dried by synthesis gas into a gasifier, a pressurization feedstock supply system including a buffer bin and a pressure lock hopper has been used. However, this pressurization feedstock supply system is problematic in that it is expensive and complicated and its operation logic must be elaborately adjusted. Particularly, when dried coal is carried to the gasifier using the pressure lock hopper, first, dried coal is transferred to the buffer bin, and is then supplied to the gasifier together with carrier gas with the dried coal being pressurized by the pressure lock hopper. In this case, the operation of a valve, the pressurization and depressurization of the pressure lock hopper, and the like must be accurately adjusted.
Further, synthesis gas is discharged from a gasifier at high temperature (1000 to 1500□), whereas synthesis gas having relatively low temperature (about 500□) may also be used in order to dry water-containing coal. For this reason, in conventional technologies, in order to supply thermal energy suitable for drying a water-containing feedstock introduced into a dryer, a heat exchanger is disposed behind a gasifier to decrease the temperature of the feedstock to the temperature of synthesis gas, and then the feedstock is recycled or is cooled to a temperature suitable for subsequent processes (for example, water-gas-shift reaction).
In this case, solid fuel dried by dryer, particularly, dried coal has higher quality than water-containing coal. Therefore, if such high-quality solid fuel (that is, dried coal) can be produced such that it is used as a feedstock for a gasification reaction as well as is applied to other uses, the added value in the total process will be increased. However, in order to produce a feedstock for a gasification reaction and high-quality solid fuel (coal) using the above-mentioned pressurization system including a buffer bin and a pressure lock hopper, it is inevitable that the size of equipment increases, and thus equipment investment cost and operation cost increase.